Lanthanide Borohydrides Research Articles

Valorization of bioresources for the production of polymer using lanthanide borohydride as catalysts

Valorization of bioresources for the production of polymer using lanthanide borohydride as catalysts

Cyclopentadienyl lanthanide borohydrides derived from the unsubstituted cyclopentadienyl ligand. Unprecedented structural diversity and ε-caprolactone polymerization

A series of mono-, bis- and tris(cyclopentadienyl) complexes of La, Nd and Tb with borohydride ligand were synthesized and structurally characterized. Various structural types of complex anions and cations containing cyclopentadienyl and borohydride ligands in the coordination sphere of Ln3+ have been studied by single-crystal X-ray diffraction. Five different structural types of Cp-lanthanide borohydrides were found for neodymium: {CpNd(BH4)2} in [(CpNd(BH4)2(THF)2] (2Nd); {Cp2Nd(BH4)2}− in [Cp2Nd(BH4)2][Na(18-crown-6)(THF)2] (3Nd); {Cp2Nd(BH4)} in [Cp2Nd(BH4)(THF)] (4Nd); {CpNd(18-crown-6)(BH4)}+ in [Cp2Nd(BH4)2]2[CpNd(18-crown-6)(BH4)][Na(18-crown-6)(THF)2] (6Nd); {Cp3Nd(BH4)}− in [Cp3Nd(BH4)][Na(18-crown-6)(THF)2] (8Nd). Complex anions {Cp2Nd(BH4)2}− and {Cp3Nd(BH4)}− are able to replace each other in the crystal lattice. Mono- and bis(cyclopentadienyl)borohydride complexes of neodymium effectively initiate ε-caprolactone polymerization.

Structural and dynamic studies of Pr(11BH4)3

Rare earth borohydrides RE (BH4)3 are studied in the context of energy storage, luminescence and magnetic applications. We have investigated the structural behavior of praseodymium borohydride Pr (11BH4)3 containing 11B isotope because of the previously reported negative thermal expansion. Differential scanning calorimetry (DSC), in-situ variable temperature synchrotron radiation powder X-ray diffraction (SR-PXD) and infrared studies reveal that Pr (11BH4)3 undergoes to a volume contraction during the phase transition from alpha α-Pr (11BH4)3 to rhombohedral r-Pr (11BH4)3 phase upon heating to 493 K. Surprisingly, the phase transition persists upon cooling at room temperature. Vibrational analysis also shows that the stretching frequency of BH4− anion does not change upon heating which indicates that the B–H bond length remains constant during the structural phase transition from α-Pr (11BH4)3 to r-Pr (11BH4)3 phase. Additionally, the energy barrier of reorientation motion of the BH4− anion in the α-phase was estimated to be ca 23 kJ/mol by quasi-elastic neutron scattering (QENS) and Raman spectroscopy.

Synthesis, Polymorphism and Thermal Decomposition Process of (n-C4H9)4NRE(BH4)4 for RE = Ho, Tm and Yb.

In total, three novel organic derivatives of lanthanide borohydrides, n-But4NRE(BH4)4 (TBAREB), RE = Ho, Tm, Yb, have been prepared utilizing mechanochemical synthesis and purified via solvent extraction. Studies by single crystal and powder X-ray diffraction (SC-XRD and PXRD) revealed that they crystalize in two polymorphic forms, α- and β-TBAREB, adopting monoclinic (P21/c) and orthorhombic (Pnna) unit cells, previously found in TBAYB and TBAScB, respectively. Thermal decomposition of these compounds has been investigated using thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) measurements, along with the analysis of the gaseous products with mass spectrometry (MS) and with analysis of the solid decomposition products with PXRD. TBAHoB and TBAYbB melt around 75 °C, which renders them new ionic liquids with relatively low melting points among borohydrides.

Electrodeposition of neodymium and dysprosium from organic electrolytes.

A new class of organic electrolytes has been developed for the electrodeposition of rare-earth metals at room temperature. These electrolytes consist of a rare-earth bis(trifluoromethylsulfonyl)imide or chloride salt and a borohydride salt, dissolved in the ether solvents 1,2-dimethoxyethane or 2-methyltetrahydrofuran. In these electrolytes, a soluble lanthanide(iii) borohydride complex [Ln(BH4)4]- is formed, which allows for the electrodeposition of neodymium- or dysprosium-containing layers. The electrochemistry of these electrolytes was characterized by cyclic voltammetry. The deposits were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray fluorescence (EDX) and X-ray photoelectron spectroscopy (XPS), and the results suggest the presence of metallic neodymium and dysprosium.

Probing the local symmetry of Tb3+ in borohydrides using luminescence spectroscopy

Rare-earth borohydrides have recently attracted a strong interest as potential hydrogen storage materials. While the local structure of the BH4− ion can be probed using vibrational spectroscopy, the local structure of the rare-earth ion can be studied using luminescence spectroscopy. This is reported here for Tb(BH4)3 and its solvate with S(CH3)2. The spectra clearly show smaller CF splittings for the unsolvated compound. These experimental data are compared with recent DFT+U calculations of the energy levels and CF splittings for Tb(BH4)3 and show that these calculations significantly overestimate the experimental results.

Borohydride as Magnetic Superexchange Pathway in Late Lanthanide Borohydrides

The magnetic characteristics of a series of borohydride‐based systems, including binary α‐/β‐Ln(BH4)3 phases (Ln = Gd, Tb, Dy, Ho, Er, and Tm) with direct Ln–HBH–Ln bridges and selected mixed‐metal systems, LiYb(BH4)4, NaYb(BH4)4, KHo(BH4)4, RbTm(BH4)4, with isolated Ln3+ ions embedded in [Ln(BH4)4]– anions are described for the first time using SQUID magnetometry and DFT+U calculations. Crystal field effects as well as the nature and strength of magnetic superexchange interactions via BH4– ligands are established based on a ligand field theory approach. We find Auzel's scalar crystal field strength parameter (Nv, the weighted quadratic mean of the ligand field parameters Bkq) for these systems to be substantial, in particular for α‐Ho(BH4)3, indicating a significant covalent component in the Ho···H bonding. The BH4– anion is capable of mediating both weakly ferro‐ or antiferromagnetic superexchange. Quantum mechanical DFT+U calculations, even when including spin‐orbit coupling effects, do not reliably describe the sign and the size of the magnetic superexchange constants in these systems.

Preparation of a series of lanthanide borohydrides and their thermal decomposition to refractory lanthanide borides

Several solvent-free borohydrides of trivalent lanthanides (Nd–Lu) and their composites with LiBH4 have been targeted utilizing mechanochemical synthesis, and physicochemically characterized. Overall, the eight new borohydride phases are reported, which include the two polymorphs of the previously elusive Eu(BH4)3, completing the series of the known inorganic borohydrides of trivalent lanthanides. Thermolysis of borohydrides yielded refractory lanthanide borides; the attempts of purification of the post-thermolysis composites were unsuccessful.

Rare Earth Borohydrides—Crystal Structures and Thermal Properties

Rare earth (RE) borohydrides have received considerable attention during the past ten years as possible hydrogen storage materials due to their relatively high gravimetric hydrogen density. This review illustrates the rich chemistry, structural diversity and thermal properties of borohydrides containing RE elements. In addition, it highlights the decomposition and rehydrogenation properties of composites containing RE-borohydrides, light-weight metal borohydrides such as LiBH4 and additives such as LiH.

In situ investigations of bimetallic potassium erbium borohydride

Research on rare earth (RE) borohydrides is increasing due to their potential as possible hydrogen storage materials and as solid-state Li-ion conductors for battery applications. In this work, we report the formation of a new bimetallic RE borohydride, KEr(BH4)4, via mechanochemical reaction between KBH4 and Er(BH4)3 in a molar ratio of 1:1. Post-annealing of the ball-milled mixture at 175 °C for 12 h under 40 bar of hydrogen resulted in an almost phase pure material. KEr(BH4)4 is isostructural to NaSc(BH4)4 with lattice parameters of a = 8.4472(8) Å, b = 12.4292(11) Å, c = 9.6252(9) Å and space group Cmcm. An amorphization of the new bimetallic phase was observed at ∼170 °C under 1 bar argon via temperature programmed photographic analysis, which is in agreement with in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as well as thermogravimetric and differential scanning calorimetry (TG-DSC). The in situ SR-PXD data did not show decomposition into the starting monometallic borohydrides.

Energy Change Regularities of Crystal Lattice of Lanthanide Borohydrides

Thermodynamic characteristics of lanthanides' borohydrides are defined by tensimetric, calorimetric and semiempirical methods. Lanthanides' borohydride crystal energy lattice is defined by compiled Born-Haber cycle and their change regularity within the whole lanthanide's row is determined.

The influence of LiH on the rehydrogenation behavior of halide free rare earth (RE) borohydrides (RE = Pr, Er).

Rare earth (RE) metal borohydrides are receiving immense consideration as possible hydrogen storage materials and solid-state Li-ion conductors. In this study, halide free Er(BH4)3 and Pr(BH4)3 have been successfully synthesized for the first time by the combination of mechanochemical milling and/or wet chemistry. Rietveld refinement of Er(BH4)3 confirmed the formation of two different Er(BH4)3 polymorphs: α-Er(BH4)3 with space group Pa3[combining macron], a = 10.76796(5) Å, and β-Er(BH4)3 in Pm3[combining macron]m with a = 5.4664(1) Å. A variety of Pr(BH4)3 phases were found after extraction with diethyl ether: α-Pr(BH4)3 in Pa3[combining macron] with a = 11.2465(1) Å, β-Pr(BH4)3 in Pm3[combining macron]m with a = 5.716(2) Å and LiPr(BH4)3Cl in I4[combining macron]3m, a = 11.5468(3) Å. Almost phase pure α-Pr(BH4)3 in Pa3[combining macron] with a = 11.2473(2) Å was also synthesized. The thermal decomposition of Er(BH4)3 and Pr(BH4)3 proceeded without the formation of crystalline products. Rehydrogenation, as such, was not successful. However, addition of LiH promoted the rehydrogenation of RE hydride phases and LiBH4 from the decomposed RE(BH4)3 samples.

Synthesis and characterization of a series of mixed-cation borohydrides of scandium: [Cat][Sc(BH4)4], [Cat] = [Me4N], [n-Bu4N], and [Ph4P

Three organic mixed-cation borohydrides of scandium: [Cat][Sc(BH4)4], [Cat]=[Me4N], [n-Bu4N], and [Ph4P], denoted 1, 2 and 3, respectively, have been prepared and characterized. Compounds 1 and 3 are isostructural to the analogous rare earth borohydrides crystallizing in Pnma and I41/a space group, respectively. Compound 2 crystallizes in Pnna space group adopting structure of [Bu4N][FeCl4], which is different from that of its yttrium analogue (P21/c). The thermal decomposition of these substances starts above ca. 175 (compound 2), 195 (3), and 215°C (1), and proceeds with evolution of volatile organic products. The thermal decomposition of 2 and 3 is preceded by melting at ca. 88 and 153°C, respectively, which renders the compound 2 an ionic liquid. The compounds 2 and 3 are well soluble in various organic solvents and can be easily purified via dichloromethane extraction.

Hydrogen storage properties of rare earth (RE) borohydrides (RE = La, Er) in composite mixtures with LiBH4 and LiH

Mixtures of 6LiBH4–RECl3–3LiH (RE=La, Er) have been produced by mechanochemical milling and their structure, thermal decomposition and reversibility have been studied. Hydrogen desorption starts around 300°C in both composites. Heating to 400°C yields LaB6, ErB4 and REH2+δ as major decomposition products. LiBH4 is destabilized by REH2+δ formed through decomposition of the parent borohydrides LiLa(BH4)3Cl and Er(BH4)3, respectively, and its hydrogen release temperature is reduced by 100°C as compared to pure ball-milled LiBH4. The lanthanum-containing composite releases 4.2wt.% H between 300 and 350°C and shows a limited reversibility of ∼20% (340°C, 10MPa) probably due to hydrogen uptake by some amorphous boron-containing phases. For 6LiBH4–ErCl3–3LiH about 3wt.% H is evolved up to 400°C. Desorption against 0.5MPa backpressure results in an increased reversibility (∼80%) as compared to vacuum (∼66%). Rehydrogenation (340°C, 10MPa) shows the formation of ErH3 and LiBH4 at drastically reduced conditions compared to pure LiBH4 (>400°C, >10MPa).

(Iminophosphoranyl)(thiophosphoranyl)methane rare-earth borohydride complexes: synthesis, structures and polymerization catalysis.

The (iminophosphoranyl)(thiophosphoranyl)methanide {CH(PPh2=NSiMe3)(PPh2=S)}(-) ligand has been used for the synthesis of divalent and trivalent rare-earth borohydride complexes. The salt metathesis of the potassium reagent [K{CH(PPh2=NSiMe3)(PPh2=S)}]2 with [Yb(BH4)2(THF)2] resulted in the divalent monoborohydride ytterbium complex [{CH(PPh2=NSiMe3)(PPh2=S)}Yb(BH4)(THF)2]. The 2D (31)P/(171)Yb HMQC-NMR spectrum clearly showed the coupling between both nuclei. The trivalent bisborohydrides [{CH(PPh2=NSiMe3)(PPh2=S)}Ln(BH4)2(THF)] (Ln = Y, Sm, Tb, Dy, Er, Yb and Lu) were obtained by reaction of [K{CH(PPh2=NSiMe3)(PPh2=S)}]2 with [Ln(BH4)3(THF)3]. All new compounds were characterized by single X-ray diffraction. The divalent and trivalent compounds were next used as initiators in the ring-opening polymerization (ROP) of ε-caprolactone (CL) and trimethylene carbonate (TMC). All complexes afforded a generally well-controlled ROP of both of these cyclic esters. High molar mass poly(ε-caprolactone) diols (Mn,NMR < 101,300 g mol(-1), ĐM = 1.44), and α,ω-dihydroxy and α-hydroxy,ω-formate telechelic poly(trimethylene carbonate)s (Mn,NMR < 20,000 g mol(-1), ĐM = 1.61) were thus synthesized under mild operating conditions.

Polymorphism and hydrogen discharge from holmium borohydride, Ho(BH4)3, and KHo(BH4)4

Holmium borohydride, Ho(BH4)3, its composites with LiBH4, and a mixed–cation derivative, K[Ho(BH4)4], have been prepared via mechanochemical reaction between HoCl3, LiBH4 (and also KBH4 in case of K[Ho(BH4)4]). These compounds are isostructural to the related rare earth borohydrides, adopting α-Y(BH4)3, β-Y(BH4)3, and Na[Sc(BH4)4]-type structures, respectively. The relative amount of α-Ho(BH4)3 and β-Ho(BH4)3 can be controlled by the composition of reagents. While β-Ho(BH4)3 has not been obtained from stoichiometric mixtures of HoCl3 and LiBH4, the excess of LiBH4 favours it as the main product. Thermal decomposition of α-Ho(BH4)3, as well as K[Ho(BH4)4] commences above 170 °C, with the fastest rate within 250–260 °C, which is slightly lower than for the corresponding borohydrides of yttrium. LiBH4 is destabilised thermally in the composites with β-Ho(BH4)3, which leads to desorption of >40% total amount of H2 below 450 °C, while in case of pure LiBH4 only <20% total amount of H2 is released in these conditions. The catalytic mechanism is as yet unknown.

β-Diketiminate Rare Earth Borohydride Complexes: Synthesis, Structure, and Catalytic Activity in the Ring-Opening Polymerization of ε-Caprolactone and Trimethylene Carbonate

The synthesis of a series of divalent and trivalent β-diketiminate borohydrides [(dipp)2NacNacLn(BH4)(THF)2] ((dipp)2NacNac = (2,6-C6H3iPr2)NC(Me)CHC(Me)N(2,6-C6H3iPr2); Ln = Sm, Eu, Yb) and [(dipp)2NacNacLn(BH4)2(THF)] (Ln = Sc, Sm, Dy, Yb, Lu) is reported. All compounds were obtained by salt metathesis in THF from [(dipp)2NacNacK] and the corresponding homoleptic divalent and trivalent borohydrides [Ln(BH4)2(THF)2] (Ln = Sm, Eu, Yb), [Sc(BH4)3(THF)2], and [Ln(BH4)3(THF)3] (Ln = Sm, Dy, Yb, Lu), respectively. The complexes were fully characterized, and their solid-state structures were established by single-crystal X-ray diffraction. In both the divalent and trivalent compounds, the BH4– groups coordinate in a κ3(H) mode to the metal. Only in the lutetium complex [(dipp)2NacNacLu(BH4)2(THF)] does one BH4– group coordinate in a κ3(H) mode, whereas the other one coordinates as κ2(H). This kind of mixed κ2/κ3(H) coordination mode is rare. The application of the divalent and trivalent compounds as initiators in the ring-opening polymerization (ROP) of ε-caprolactone (CL) and trimethylene carbonate (TMC) was investigated. All complexes afforded a generally well-controlled ROP of both of these cyclic esters. High molar mass poly(ε-caprolactone) diols (Mn,NMR < 92 700 g mol–1, ĐM = 1.51) and α-hydroxy,ω-formate telechelic poly(trimethylene carbonate)s (Mn,NMR < 16 000 g mol–1, ĐM = 1.59) were thus synthesized under mild operating conditions.

PMLABe Diol Synthesized by Ring-Opening Polymerization of Racemic Benzyl β-Malolactonate Initiated by Rare-Earth Trisborohydride Complexes: An Experimental and DFT Study

Polymer diols are a class of polymeric building blocks of high interest for the synthesis of complex macromolecular edifices. Rare-earth borohydride complexes are known as efficient initiators for the ring-opening polymerization (ROP) of cyclic esters, directly affording α,ω-dihydroxy-telechelic polyesters. Here, were report the direct synthesis of poly(benzyl β-malolactonate) (PMLABe) diols, from the ROP of racemic (benzyl β-malolactonate) (rac-MLABe), a valuable and renewable monomer, initiated by the homoleptic [Ln(BH4 )3 (thf)3 ] (Ln=La, Nd, and Sm) complexes. These initiators enabled the controlled ROP of this β-lactone, affording well-defined syndiotactic-enriched (Pr ≈0.83) PMLABes (Mn up to 21 300 g mol(-1) , ÐM ≈1.5) as evidenced by size exclusion chromatography, (1) H and (13) C NMR spectroscopy, and MALDI-ToF mass spectrometry analyses. The first and second insertions of rac-MLABe, as assessed by DFT calculations, revealed more favorable stationary front-side than migratory back-side insertions, the thermodynamically and kinetically competitive ROP on two distinct arms with that on a one arm-only, and the thermodynamically slightly favored formation of syndiotactic-enriched PMLABes.

Controlled Polymerization of N-Substituted Glycine N-Thiocarboxyanhydrides Initiated by Rare Earth Borohydrides toward Hydrophilic and Hydrophobic Polypeptoids

N-substituted glycine N-thiocarboxyanhydrides (NTAs) are alternative monomers to prepare polypeptoids with large-scale producing potential compared to the corresponding N-carboxyanhydrides (NCAs) due to their easily synthetic approach and stability during purification and storage. Novel monomer N-butylglycine NTA (NBG-NTA) has been synthesized and well characterized for the first time. Rare earth borohydrides [RE(BH4)3(THF)3, RE = Sc, Y, La, Nd, Dy, and Lu] have been first applied in the polymerization of sarcosine NTA (Sar-NTA) and NBG-NTA to achieve high molecular weight (MW) hydrophilic and hydrophobic polypeptoids. Polysarcosines (PSars), poly(N-butylglycine)s (PNBGs), and their copolymers with high yields, high MWs, and moderate MW distributions are synthesized at 60 °C by using RE(BH4)3(THF)3 initiators. MWs of polypeptoids are controlled by feed molar ratios. For instance, PSar with an absolute Mn of 27.7 kDa (DP = 390) and PDI of 1.14 is produced successfully from Sar-NTA. Thermoresponsive random copolypeptoids poly(sarcosine-r-N-butylglycine)s [P(Sar-r-NBG)s] have reversible phase transitions (cloud point temperature) in aqueous solution and minimal cytotoxicity comparable to PEG and PSar, which is promising in various biomedical and biotechnological applications. Thermal properties of homo- and co-polypeptoids are investigated by TGA and DSC measurements.

Novel lanthanide borohydrides: magnetism of all flavours

The rare-earth metals have high magnetic moments and a diverse range of magnetic structures. However, due to the inner-transition nature of lanthanide elements, the valence f orbitals of their trivalent cations usually do not mix substantially with the ligands' orbitals in the chemical compounds. The majority of lanthanide compounds is thus characterized by a rather ionic metal–ligand bonding and is hosting only weak crystal field effects. Several exceptions known encompass the valence fluctuation systems consisting of Sm, Ey, Tm or Yb combined with less electronegative nonmetal ligands (Si, S, Se, B etc.) or metals (Murani 2003 and references therein). This important class of lanthanide compounds for which crystal field effects are strong includes the classical systems: Yb3Si5 (Iandelli et al., 1979), YbB12 (Altshuler et al., 1998), and Yb3H8 (Drulis et al., 1999) . Even elemental Yb and Eu metals show valence transition at elevated pressure from di- to trivalent (Takemura &amp; Syassen, 1985). These valence fluctuations are typically accompanied by electric resistivity changes: Ln(2+) → Ln(3+) + e–. Lanthanide borohydrides, Ln(BH4)3, constitute a rather poorly explored and novel group of compounds (Olsen et al., 2014). They are conveniently prepared via mechanochemical synthesis approach (high-energy milling). Quasi-ternary alkali metal-lanthanide borohydrides, MLn(BH4)4, are also available using this synthetic procedure (Wegner et al., 2013 [1] &amp; Wegner et al., 2014 [2]). Here we explore for the first time the magnetic properties of Ln(BH4)3 and MLn(BH4)4 compounds, with particular emphasis on the thermally unstable systems (Ln= Sm, Yb and Eu) as contrasted with the reference case of much more thermally stable derivatives of ordinary lanthanides (Ln = Ho). We show that remarkably strong mixing of Ln(4f) and H(1s) states which causes thermal instability: Ln(3+) + BH4–→ Ln(2+) + BH4· leads in some cases to strong magnetic superexchange interactions between Ln(3+) centers [3].